Quick answer
Hyperthyroidism in cats is an overactive thyroid gland, usually caused by a benign growth, and it's one of the most common hormone problems in cats over 10. The classic sign is weight loss despite a big appetite. It's very treatable with daily medication, a special diet, surgery or radioactive iodine, which cures most cats with a single treatment.
What is hyperthyroidism in cats?
Hyperthyroidism means the thyroid glands in the neck make too much thyroid hormone (T4). This hormone controls metabolism, so an overactive thyroid speeds up nearly every organ, including the heart.
In about 97% of cats the cause is a benign growth or enlargement of the thyroid tissue, according to the Merck Veterinary Manual. Both glands are affected in about 70% of cats. Thyroid cancer is rare.
It’s mostly a disease of middle-aged and older cats, usually over 10. It’s one of the most common hormone problems vets see in senior cats.
What are the signs of an overactive thyroid in cats?
Signs come on slowly, so they’re easy to put down to old age at first.
- Weight loss even though your cat is eating more, sometimes much more
- Drinking and peeing more (see cat drinking a lot of water)
- Restlessness, hyperactivity or loud yowling, often at night
- Vomiting or diarrhea
- An unkempt, greasy or matted coat
- A fast heart rate, which your vet may hear as a murmur
- A lump in the neck your vet can feel
A small number of cats become quiet and lose their appetite instead. Any unexplained weight loss in an older cat is worth a vet visit. Our should I take my cat to the vet? checker can help you judge how soon.
Hyperthyroidism shares signs with cat kidney disease and diabetes, and older cats can have more than one, so your vet will usually check for all three.
How is hyperthyroidism diagnosed?
Your vet will examine your cat, feel the neck for an enlarged thyroid and check heart rate and blood pressure.
- Total T4 blood test: a high T4 confirms the diagnosis in most cats.
- Free T4: helps when T4 is in the high-normal range but signs fit. Some cats have a normal T4 early on or when another illness lowers it.
- Blood panel and urinalysis: to check the kidneys, liver and other organs.
- Blood pressure: high blood pressure is common and needs its own treatment.
An overactive thyroid increases blood flow to the kidneys, which can hide kidney disease. That’s why your vet will recheck kidney values after treatment starts.
What are the treatment options for hyperthyroidism in cats?
There are four main options. Your vet will help you choose based on your cat’s age, other health problems, how easy your cat is to medicate and your budget.
| Treatment | How it works | Cure? | Main downsides |
|---|---|---|---|
| Methimazole | Daily pill or gel rubbed on the ear, blocks hormone production | No, lifelong | Daily dosing, side effects in some cats |
| Radioactive iodine | One injection destroys the overactive tissue | Yes, about 95% of cats | Specialist center, hospital stay, higher upfront cost |
| Iodine-restricted diet | Prescription food with too little iodine to make excess hormone | No, lifelong | Must eat only that food, works best in mild cases |
| Surgery (thyroidectomy) | Removes the affected gland | Often | Anesthesia risk, can affect calcium levels, can recur |
Methimazole
Methimazole is the most common medication. It’s given as a tablet, liquid or a gel applied to the inner ear, usually every day for life. Your vet will set the dose, how often to give it, and adjust it based on T4 rechecks.
Most cats do well on it. Vomiting and poor appetite are the most common side effects, and the ear gel can help. Rarer problems include facial itching and scratching, liver changes and low blood cell counts, so regular bloodwork matters. Call your vet if your cat stops eating, turns yellow, bruises or scratches its face raw.
Radioactive iodine
Radioactive iodine is considered the treatment of choice by Cornell and the Merck Veterinary Manual, because it cures about 95% of cats with a single injection and avoids daily pills. Your cat stays at a licensed center for several days, sometimes longer depending on state rules, until radiation levels drop. You’ll get home-care instructions for the first couple of weeks.
A few cats develop an underactive thyroid afterward, which is managed with supplements.
Diet and surgery
A prescription iodine-restricted diet works because the thyroid needs iodine to make hormone. Most cats reach normal levels within 8 to 12 weeks, according to the Merck Veterinary Manual, but only if they eat nothing else. No treats, no hunting and no sharing food with other cats. It suits indoor, single-cat homes best.
Surgery to remove the affected gland can be curative, but it needs general anesthesia, which is riskier in older cats with heart changes. The small parathyroid glands next to the thyroid control calcium, and damage to them can cause low calcium after surgery. Radioactive iodine has largely replaced surgery where it’s available.
Can I try a trial of methimazole first?
Yes, and many vets recommend it. A few weeks on methimazole shows how the kidneys cope once thyroid levels come down. If kidney values stay stable, you can then choose a permanent option like radioactive iodine with more confidence.
How much does it cost to treat hyperthyroidism?
Costs depend on the option you choose. Radioactive iodine costs more upfront but can cost less over several years than lifelong medication and rechecks. These are typical US ranges.
| Item | Typical cost |
|---|---|
| Diagnosis (exam, T4, panel, urinalysis) | $250 – $500 |
| Methimazole (per month) | $20 – $60 |
| T4 recheck | $60 – $150 |
| Prescription iodine-restricted diet (per month) | $50 – $110 |
| Radioactive iodine treatment | $1,200 – $2,500 |
| Surgery | $1,000 – $2,500 |
For routine visit prices, see how much a cat vet visit costs.
What is life like for a cat after treatment?
Most cats regain weight, calm down and look like themselves again within a few weeks to months of starting treatment. On medication or diet, expect T4 rechecks every few weeks at first, then every 3 to 6 months once stable.
Keep a close eye on kidney health, especially in the first months. Signs to report include new weight loss, drinking more, vomiting or poor appetite. For other conditions common in older cats, browse our cat health problems guides.
Frequently asked questions
How long can a cat live with hyperthyroidism?
With treatment, many cats live for years with a good quality of life. The outlook depends mostly on age and whether the cat also has kidney or heart disease.
Is hyperthyroidism in cats cancer?
Almost never. About 97 to 98% of cases are caused by a benign (non-cancerous) growth of the thyroid. Thyroid cancer causes only a small minority of cases.
What happens if hyperthyroidism is left untreated?
Untreated cats keep losing weight and can develop heart disease and high blood pressure, which can damage the eyes, kidneys and brain. It shortens life, so it's worth treating.
Sources
Written by the CatsMed Team from the sources above. This guide isn’t reviewed by a veterinarian and isn’t a substitute for one. See how we research.
